1. Field of the Invention.
This invention relates to a process for making thermal conductivity measurements in borehole casing.
2. Description of the Prior Art.
So far as Applicant is aware, there is no presently available well logging method for making in situ measurements of the thermal conductivity of well casing. The closest prior art found concerns a system for making thermal conductivity measurements in a dry borehole as, for example, in U.S. Pat. No. 3,668,927. This patent teaches a logging probe having a temperature sensor, a heater and expander means inserted into a borehole. The expander means is actuated so that the temperature sensor contacts the side wall. The heater then heats the sensor, and temperature measurements are made at predetermined time intervals. The slope of the curve relating temperature with time (plotted logarithmically) is linear and proportional to the thermal conductivity of the earth. Such a device requires that a sensing element be driven into the side wall of the borehole and left in that position for a period of time sufficient to generate data to establish the curve after which relocation at a different borehole position may be accomplished.
A specific need for a method of the character to be described in this application is present in permafrost wells. It has been found that the production of a well drilled through a permafrost layer causes thawing radially outward from the well-bore because of the internal heat generated within the well. When the well is not operating, the permafrost may refreeze around the well casing. The external pressures on the casing thus developed may exceed its collapse rating. Thawing also may result in subsidence of the permafrost layer with consequent damage to the well casing.
In order to minimize the thawing effect upon the permafrost as described, it is necessary to insulate the casing in some manner. For example, insulation may be introduced in the annular space separating a doubled wall string of production casing. In accordance with known techniques, this insulation may be in the form of a gel or an evacuated space.
It is desirable, therefore, to devise a method whereby the effective thermal conductivity of well casing, particularly in permafrost, may be quickly and accurately determined. This will enable location of insulation breakdown, liquid leaks, casing collapse, and peripheral contact between casing strings, etc., which affect conductivity. Thus, the invention may help to locate high conductivity zones where permafrost thaw may occur during production. Such zones may then be monitored for subsidence or refreeze problems.
The method to be described herein may find secondary use in determining the thermal conductivity of a borehole before installation of the casing. This type of measurement may thus help in determining the actual amount of insulation needed on the casing. If this measurement is combined with measurement by conventional means of temperature change in the borehole with increasing depth, it may also permit calculation of actual earth heat flux for use in predicting the actual depth of permafrost.
It is, therefore, a general object of this invention to provide a novel method for in situ measurement of thermal conductivity of a wellbore casing.
It is a further object of this invention to provide a method for measurement of the insulation quality of permafrost well casing thermal conductivity that is simple in operation and yields its information rapidly.
It is yet another object of this invention to provide a novel method for measurement of thermal conductivity of a bore-hole prior to installation of casing therein.
It is still another object of this invention to provide a novel method for calculating earth heat flux in the vicinity of a borehole.